This excerpt taken from the WBMD DEF 14A filed Nov 5, 2008.
Revocability of Proxies
Submitting a proxy on the enclosed form does not preclude a WebMD stockholder of record from voting in person at the WebMD Annual Meeting. A WebMD stockholder of record may revoke a proxy at any time before it is voted by taking any of the following actions:
WebMD stockholders whose shares are held in street name should contact their broker, bank or nominee for instructions regarding voting at the WebMD Annual Meeting or revoking previously submitted instructions regarding how their shares are to be voted.